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Chapter Goals
• • •

Discuss the history and development of the X.25 protocol. Describe the basic functions and components of X.25. Describe the frame formats of X.25.

X.25 is an International Telecommunication Union–Telecommunication Standardization Sector (ITU-T) protocol standard for WAN communications that defines how connections between userdevices and network devices are established and maintained. X.25 is designed to operate effectively regardless of the type of systems connected to the network. It is typically used in the packet-switched networks (PSNs) of common carriers, such as the telephone companies. Subscribers are charged based on their use of the network. The development of the X.25 standard was initiated by the commoncarriers in the 1970s. At that time, there was a need for WAN protocols capable of providing connectivity across public data networks (PDNs). X.25 is now administered as an international standard by the ITU-T.

X.25 Devices and Protocol Operation
X.25 network devices fall into three general categories: data terminal equipment (DTE), data circuit-terminating equipment (DCE), and packet-switchingexchange (PSE). Data terminal equipment devices are end systems that communicate across the X.25 network. They are usually terminals, personal computers, or network hosts, and are located on the premises of individual subscribers. DCE devices are communications devices, such as modems and packet switches, that provide the interface between DTE devices and a PSE, and are generally located in thecarrier’s facilities. PSEs are switches that compose the bulk of the carrier’s network. They transfer data from one DTE device to another through the X.25 PSN. Figure 17-1 illustrates the relationships among the three types of X.25 network devices.

Internetworking Technologies Handbook 1-58705-001-3


Chapter 17 X.25 Devices and Protocol Operation


Figure 17-1 DTEs, DCEs, andPSEs Make Up an X.25 Network
Personal computer X.25 WAN


PSE Modem Switch DCE

Network host


Packet Assembler/Disassembler
The packet assembler/disassembler (PAD) is a device commonly found in X.25 networks. PADs are used when a DTE device, such as a character-mode terminal, is too simple to implement the full X.25 functionality. The PAD is located between a DTE device and a DCEdevice, and it performs three primary functions: buffering (storing data until a device is ready to process it), packet assembly, and packet disassembly. The PAD buffers data sent to or from the DTE device. It also assembles outgoing data into packets and forwards them to the DCE device. (This includes adding an X.25 header.) Finally, the PAD disassembles incoming packets before forwarding thedata to the DTE. (This includes removing the X.25 header.) Figure 17-2 illustrates the basic operation of the PAD when receiving packets from the X.25 WAN.

Internetworking Technologies Handbook



Chapter 17

X.25 X.25 Devices and Protocol Operation

Figure 17-2 The PAD Buffers, Assembles, and Disassembles Data Packets


PAD X.25


Assembly/disassembly Data


X.25 Session Establishment
X.25 sessions are established when one DTE device contacts another to request a communication session. The DTE device that receives the request can either accept or refuse the connection. If the request is accepted, the two systems begin full-duplex information transfer. Either DTE device can terminate the connection. After the session isterminated, any further communication requires the establishment of a new session.

X.25 Virtual Circuits
A virtual circuit is a logical connection created to ensure reliable communication between two network devices. A virtual circuit denotes the existence of a logical, bidirectional path from one DTE device to another across an X.25 network. Physically, the connection can pass through any number of...
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